8. Waste Heat Recovery

177Bureau of Energy Efficiency

8.4 Development of a Waste Heat Recovery System

Understanding the process

Understanding the process is essential for development of Waste Heat Recovery system. This can be

accomplished by reviewing the process flow sheets, layout diagrams, piping isometrics, electrical

and instrumentation cable ducting etc. Detail review of these documents will help in identifying:

a) Sources and uses of waste heat

b) Upset conditions occurring in the plant due to heat recovery

c) Availability of space

d) Any other constraint, such as dew point occurring in an equipments etc.

After identifying source of waste heat and the possible use of it, the next step is to select

suitable heat recovery system and equipments to recover and utilise the same.

Economic Evaluation of Waste Heat Recovery System

It is necessary to evaluate the selected waste heat recovery system on the basis of financial

analysis such as investment, depreciation, payback period, rate of return etc. In addition the

advice of experienced consultants and suppliers must be obtained for rational decision.

Next section gives a brief description of common heat recovery devices available commer-

cially and its typical industrial applications.

8.5 Commercial Waste Heat Recovery Devices

Recuperators

In a recuperator, heat exchange takes

place between the flue gases and the

air through metallic or ceramic

walls. Duct or tubes carry the air for

combustion to be pre-heated, the

other side contains the waste heat

stream. A recuperator for recovering

waste heat from flue gases is shown

in Figure 8.1.

The simplest configuration for a

recuperator is the metallic radiation

recuperator, which consists of two

concentric lengths of metal tubing as

shown in Figure 8.2.

The inner tube carries the hot

exhaust gases while the external annulus carries the combustion air from the atmosphere to

the air inlets of the furnace burners. The hot gases are cooled by the incoming combustion

air which now carries additional energy into the combustion chamber. This is energy which

does not have to be supplied by the fuel; consequently, less fuel is burned for a given

furnace loading. The saving in fuel also means a decrease in combustion air and therefore

Figure 8.1 Waste Heat Recovery using Recuperator